1. Field of the Ivention
The invention relates to an optical inspection system.
2. Discussion of Prior Art
In a number of applications it is desirable to inspect an object to ensure that it has a required surface structure. For example, it is often necessary to inspect the surface structures of manufactured items on a production line to ensure that they meet a quality criterion. The surface structure of a test object may be inspected by comparing it to a reference object. having a required surface structure. Conventionally, such inspection may be performed using a camera interfaced with a computer to capture and compare images of the reference object and the test object.
This technique has the disadvantages that a computer is required, that considerable computer processing may be required If the surface structures of the objects concerned are complex and that considerable effort or expenditure is required to develop or purchase the software necessary to compare captured images. Another disadvantage of this technique is that there is a delay involved in obtaining the results of inspection. In a production process it is important to correct errors quickly in order to prevent a significant quantity of sub-standard products being output from the process. A further disadvantage of this technique is that it becomes increasingly difficult and time-consuming with increasing spatial frequency of features of objects to be inspected. For example, in the field of semiconductor integrated circuits, the everincreasing scale of integration is resulting in increasingly dense circuitry.
An alternative method for inspecting surface structure of objects involves optical diffraction. In this method, an optical detector interfaced with a computer is used to capture and compare respective diffraction patterns of the surface structures of a reference object and a test object. However this method also has the disadvantages described above. Aftematively, the respective diffraction patterns may be compared by use of a spatial filter. This involves making a spatial filter based on the diffraction pattern of the surface structure of the reference object and forming the diffraction pattern of the surface structure of the test object on the spatial filter. If the surface structure of the test object is identical to that of the reference object, no light passes through the spatial filter. However, this inspection method gives spurious results if the spatial filter is not accurately aligned. In addition, considerable effort has to be expended in producing bespoke spatial filters for different manufacturing processes.
Another prior art method which may be used to compare the surface structure of objects is holography. In this method, a hologram of the reference object is recorded on a photographic plate. The reference object is then replaced with the test object light passing through the photographic plate indicates that the test object has a different surface structure to that of the reference object, i.e. that the test object is defective. Again, spurious results are extremely difficult to avoid because the test object must be positioned exactly in the place of the reference object. A good quality test object displaced by a fraction of a wavelength may be rejected as defective because some light may pass through the photographic plate.
A further technique which may be used to compare reference and test objects is holographic correlation, which is described, for example, in U.S. Pat. Nos. 5,282,067 and 4,212,636. In this technique, a hologram is generated using preference object and diffracted light from a test object is passed through the hologram to produce an optical field which is used to generate a spatial cross-correlation function representing the degree to which the test and reference objects are alike. Systems for implementing holographic correlation are complex because they require means for generating holograms. Also, the spatial cross-correlation function requires interpretation in order to discern the degree to which test and reference objects are alike.
In another optical correlation technique, described in published UK patent application GB 2 165 983 A, light diffracted from reference and test objects is used to generate a grating structure within a nonlinear crystal. The grating structure is representative of the product of two Fourier transforms; one Fourier transform represents light diffracted from the reference object and the other represents light diffracted by the test object light diffracted from the grating is used to generate a spatial cross-correlation function representing the degree to which the test and reference objects are alike. Systems for implementing this technique are complex. Also, the spatial cross-correlation function requires interpretation in order to discern the degree to which test and reference objects are alike.
It is also desirable in certain circumstances to detect and image the edges of solid objects or refractive index discontinuities in transparent objects. For example, edge detection allows a test object to be examined for quality on the basis of its edge structure only, allowing objects with different surface features but the same required edge structure to be examined in a single process. It may also be required to examine the quality a series of transparent test objects in a production process.
Forming an image of the edge of a solid object only is also presently very difficult For example, a shadow formed by the edge of an object is not an image purely of the edge of the object
Optical inspection of a transparent object having a refractive index discontinuity by comparing it to a reference object is also currently very difficult. Although Schlieren photography is a known technique for refractive index mapping, it relies on spatial filtering. Schlieren photography is described in “Optics” by E. Hecht (second edition, 1987, Addison-Wesley, ISBN 0-201-11611-1) pp 576 and 577. If this method were to be used to compare reference and test objects, a spatial filter would be required. This would present the alignment difficulties described above in relation to the inspection of objects using diffraction. Again, bespoke spatial filters would be required for different manufacturing processes.